Clicking a button or link that says "I agree" will let you enter a porn website under whatever conditions that you have "agreed" to (e.g. you are 18 or older).

It is supposed to be a deterrent to prevent minors from accessing pornography, often using scary-sounding words like "attest" and "perjury". But, like in the example below, it's rarely effective.

Mistress Victoria's Totally Kinky Bondage Site: "By clicking 'I agree' below, you hereby attest under penalty of perjury that you are at least 18 years of age or older, or that you are of the age of majority in your jurisdiction, and that pornography is legal in your area, and that you are not offended by hot kinky femdom porn, and that it is not considered obscene in your area, and that ...

16-year-old: "Yeah, yeah whatever. So if I click 'I agree' I get in right? Sweet!..." (Clicks on button)